1. Field of the Invention
The present invention relates to an oxygen-concentration detecting apparatus and method using an oxygen sensor which is used for generating a current signal representing the concentration of oxygen in gas whose oxygen-concentration is to be measured when a voltage is applied to the oxygen sensor. The oxygen-concentration detecting apparatus typically serves as an air-fuel ratio detecting apparatus used in implementation of air-fuel ratio feedback control of an engine employed in a vehicle.
2. Description of Related Art
In recent years, there are a demand for an improved control accuracy and a demand for a transition to lean air-fuel mixture combustion in air-fuel ratio feedback control of an engine employed in a vehicle. In order to respond to these demands, a linear air-fuel ratio sensor (an oxygen sensor) for detecting the air-fuel ratio of mixed air supplied to the engine (oxygen-concentration in exhausted gas) linearly over a wide zone as well as an air-fuel ratio detecting apparatus (an oxygen-concentration detecting apparatus) are implemented into products. In an air-fuel ratio sensor of a limit-current type, a typical air-fuel ratio sensor, a zone for detecting a limit-current is shifted in accordance with the air-fuel ratio (the oxygen-concentration) as is generally known. In detail, as shown in a V-I (voltage-current) characteristic diagram of FIG. 21, the zone for detecting a limit-current comprises straight line segments parallel to a V axis. As shown in the figure, the farther the air-fuel ratio moves to the lean zone, the more the zone for detecting a limit-current is shifted to the positive-voltage side. On the other hand, the farther the air-fuel ratio moves to the rich side, the more the zone for detecting a limit-current is shifted to the negative-voltage side. As a result, if the applied voltage is firmly set at a fixed value at the time the air-fuel ratio changes, it would be impossible to detect an air-fuel ratio with a high degree of accuracy by using the zone for detecting a limit-current (the zone comprising straight line segments parallel to the V axis).
In order to solve this problem, in a conventional ordinary air-fuel ratio detecting apparatus (oxygen-concentration detecting apparatus), the voltage applied to the air-fuel ratio sensor is varied from time to time in accordance with the air-fuel ratio, that is, in accordance with the sensor-current as is the case with those disclosed in Japanese Patent Laid-open Nos. Sho61-237047 and Sho61-280560. In this case, the applied voltage is controlled according to a characteristic line Lx shown in FIG. 21. By controlling the applied voltage in this way, a desired sensor-current, that is, a limit-current can always be obtained. Thus, the characteristic line Lx is given as a linear straight line of a positive characteristic (a characteristic rising to the right) in the V-I coordinate system.
However, the conventional technology described above raises the following problems. In an ordinary air-fuel ratio detecting apparatus, the air-fuel ratio detection zone is set in a predetermined zone. In the case of the V-I characteristic shown in FIG. 21, the air-fuel ratio detection zone covers the A/F ratio zone 12 to 18. By setting the applied voltage in accordance with the characteristic line Lx in this air-fuel ratio detection zone, the air-fuel ratio can be properly detected. For an air-fuel ratio outside the air-fuel ratio detection zone, however, a high voltage on the positive or negative side is applied, giving rise to a problem that an excessively large sensor-current flows, accompanying the high applied voltage. In such a case, a large current flows into a bias control circuit for generating a voltage to be applied to the air-fuel ratio sensor, raising a problem such as dissipation of heat in the circuit.
In more detail, in the event of a fuel cut-off, that is, when the operation to supply fuel to the engine is halted while the engine is operating, for example, the air-fuel ratio shifts far to the lean zone from the air-fuel ratio detection zone. At that time, if a voltage set in accordance with the characteristic line Lx is applied to the air-fuel ratio sensor, an excessively large sensor-current flows, accompanying the applied voltage. In addition, when the amount of injected fuel is raised in response to an increase in load to a heavy one like during acceleration of the vehicle, the air-fuel ratio shifts far to the rich zone. Also in such a case, there is raised a problem that an excessively large sensor-current flows as well.